use std::sync::Once;
static mut GLOBAL_CONFIG: Option<String> = None;
static INIT: Once = Once::new();
fn init_global_config() {
    unsafe {
        GLOBAL_CONFIG = Some("Initialized global configuration".to_string());
    }
}
fn get_global_config() -> &'static str {
    INIT.call_once(|| init_global_config());
    unsafe { GLOBAL_CONFIG.as_ref().unwrap() }
}
fn main() {
    println!("{}", get_global_config());
    println!("{}", get_global_config()); //不会被初始化？
}
